Conventionally, in a lithography process for manufacturing electronic devices such as a semiconductor device (such as an integrated circuit), a liquid crystal display device or the like, a reduction projection exposure apparatus by the step-and-repeat method that transfers an image of a pattern of a mask or a reticle (hereinafter generally referred to as a ‘reticle’) onto each of a plurality of shot areas on a photosensitive substrate such as a wafer coated with a resist (photosensitive agent), a glass plate, or the like (hereinafter generally referred to as a ‘substrate’ or a ‘wafer’) via a projection optical system, or a projection exposure apparatus by the step-and-scan method (the so-called scanning stepper (also referred to as a scanner)) are mainly used.
Resolution R of the projection optical system that the projection exposure apparatus has can be expressed as in equation (1) below by Rayleigh's formula.R=k1·λ/NA  (1)
In this case, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 is a process factor. According to equation (1), resolution R becomes higher when the exposure wavelength used (the wavelength of the exposure light) becomes shorter or when the numerical aperture or the projection optical system (NA) becomes larger. Therefore, as the integrated circuit becomes finer, the exposure wavelength used in the projection exposure apparatus is becoming shorter year by year, and nowadays, exposure apparatus that uses the ArF excimer laser (wavelength 193 mm) whose wavelength is shorter than the KrF excimer laser (wavelength 248 nm) is also put to practical use. Further, the numerical aperture of the projection optical system is gradually increasing.
When performing exposure, the depth of focus (DOF) is also important as well as the resolution. Depth of focus δ can be expressed as in equation (2) below.δ=k2·λ/NA2  (2)
In this case, k2 is a process factor. From equations (1) and (2), it can be seen that when exposure wavelength λ is shortened and numerical aperture NA is increased (a larger NA) in order to increase resolution R, depth of focus δ becomes narrower. In the projection exposure apparatus, when exposure is performed, because the surface of the wafer is made to conform to the image plane of the projection optical system, depth of focus δ should preferably be wide to some extent.
However, due to the shorter wavelength of the exposure light and the larger numerical aperture of the projection optical system described above, the depth of focus is becoming narrower. Further, the exposure wavelength is presumed to be much shorter in the future; however, in such a case, the depth of focus may become so small that focus margin shortage may occur during the exposure operation.
Therefore, as a method of substantially shortening the exposure wavelength while increasing (widening) the depth of focus when compared with the depth of focus in the air, the exposure apparatus that uses the immersion method is recently gathering attention. As such an exposure apparatus using the immersion method, the apparatus that performs exposure in a state where the space between the lower surface of the projection optical system and the wafer surface is locally filled with liquid such as water or an organic solvent is known (for example, refer to Patent Document 1 below). According to the exposure apparatus of Patent Document 1, the resolution can be improved by making use of the fact that the wavelength of the exposure light in the liquid becomes 1/n of the wavelength in the air (n is the refractive index of the liquid which is normally around 1.2 to 1.6), and the depth of focus can be also substantially increased n times when compared with the case where the same resolution is obtained by a projection optical system (supposing that such a projection optical system can be made) that does not employ the immersion method. That is, the depth of focus can be substantially increased n times than in the air.
However, in the exposure apparatus according to Patent Document 1, the liquid has to be recovered once at the point before the wafer stage moves away from under the projection optical system during wafer exchange, so as to change the state of the space between the lower surface of the projection optical system and the wafer surface from a wet state to a dry state. However, when the recovery and the supply of the liquid is performed each time the wafer is exchanged, it is certain that the time required for the recovery and supply of the liquid will cause a decrease in throughput of the exposure apparatus.
Further, when the optical path space of the projection optical system on the image plane side is changed from the wet state into the dry state in the manner described above, in the case the dry state continues, water stains (water marks) may be generated on the surface of the optical member constituting the projection optical system on the lowest end, which is also referred to as a front (lens) (such as a lens or a glass plate; hereinafter referred to as a ‘tip lens’). Further, in the case an optical member (e.g. a prism or the like), which is a member configuring an autofocus mechanism, is arranged in the vicinity of the tip lens, water stains (water marks) may be generated on the surface of the optical member configuring the autofocus mechanism. This water stain generation may lead to a decrease in transmittance of the projection optical system or may be the cause of flare, and furthermore it may be a cause of deterioration in other image-forming performances in the projection optical system. Further, in the case water marks are generated on the prism or the like referred to above, there was the risk of the plane conforming accuracy decreasing when the surface of the wafer was made to conform to the image plane of the projection optical system. Further, when many water marks are generated, the tip lens or the optical member has to be replaced, however, the time required for the replacement also becomes the cause of decreasing the operation rate of the exposure apparatus.
In the description, the stains that are formed on the tip lens or the like also in the case of using liquid other than water will also be referred to as water stains (water marks).    Patent Document 1: the Pamphlet of International Publication Number WO99/49504